1================================================================
2Documentation for Kdump - The kexec-based Crash Dumping Solution
3================================================================
4 5This document includes overview, setup and installation, and analysis
6information.
7 8Overview
9========
10 11Kdump uses kexec to quickly boot to a dump-capture kernel whenever a
12dump of the system kernel's memory needs to be taken (for example, when
13the system panics). The system kernel's memory image is preserved across
14the reboot and is accessible to the dump-capture kernel.
15 16You can use common commands, such as cp and scp, to copy the
17memory image to a dump file on the local disk, or across the network to
18a remote system.
19 20Kdump and kexec are currently supported on the x86, x86_64, ppc64 and ia64
21architectures.
22 23When the system kernel boots, it reserves a small section of memory for
24the dump-capture kernel. This ensures that ongoing Direct Memory Access
25(DMA) from the system kernel does not corrupt the dump-capture kernel.
26The kexec -p command loads the dump-capture kernel into this reserved
27memory.
28 29On x86 machines, the first 640 KB of physical memory is needed to boot,
30regardless of where the kernel loads. Therefore, kexec backs up this
31region just before rebooting into the dump-capture kernel.
32 33Similarly on PPC64 machines first 32KB of physical memory is needed for
34booting regardless of where the kernel is loaded and to support 64K page
35size kexec backs up the first 64KB memory.
36 37All of the necessary information about the system kernel's core image is
38encoded in the ELF format, and stored in a reserved area of memory
39before a crash. The physical address of the start of the ELF header is
40passed to the dump-capture kernel through the elfcorehdr= boot
41parameter.
42 43With the dump-capture kernel, you can access the memory image, or "old
44memory," in two ways:
45 46- Through a /dev/oldmem device interface. A capture utility can read the
47 device file and write out the memory in raw format. This is a raw dump
48 of memory. Analysis and capture tools must be intelligent enough to
49 determine where to look for the right information.
50 51- Through /proc/vmcore. This exports the dump as an ELF-format file that
52 you can write out using file copy commands such as cp or scp. Further,
53 you can use analysis tools such as the GNU Debugger (GDB) and the Crash
54 tool to debug the dump file. This method ensures that the dump pages are
55 correctly ordered.
56 57 58Setup and Installation
59======================
60 61Install kexec-tools
62-------------------
63 641) Login as the root user.
65 662) Download the kexec-tools user-space package from the following URL:
67 68http://www.kernel.org/pub/linux/kernel/people/horms/kexec-tools/kexec-tools.tar.gz 69 70This is a symlink to the latest version.
71 72The latest kexec-tools git tree is available at:
73 74git://git.kernel.org/pub/scm/linux/kernel/git/horms/kexec-tools.git
75or
76http://www.kernel.org/git/?p=linux/kernel/git/horms/kexec-tools.git 77 78More information about kexec-tools can be found at
79http://www.kernel.org/pub/linux/kernel/people/horms/kexec-tools/README.html 80 813) Unpack the tarball with the tar command, as follows:
82 83 tar xvpzf kexec-tools.tar.gz
84 854) Change to the kexec-tools directory, as follows:
86 87 cd kexec-tools-VERSION
88 895) Configure the package, as follows:
90 91 ./configure
92 936) Compile the package, as follows:
94 95 make
96 977) Install the package, as follows:
98 99 make install
100 101 102Build the system and dump-capture kernels
103-----------------------------------------
104There are two possible methods of using Kdump.
105 1061) Build a separate custom dump-capture kernel for capturing the
107 kernel core dump.
108 1092) Or use the system kernel binary itself as dump-capture kernel and there is
110 no need to build a separate dump-capture kernel. This is possible
111 only with the architectures which support a relocatable kernel. As
112 of today, i386, x86_64, ppc64 and ia64 architectures support relocatable
113 kernel.
114 115Building a relocatable kernel is advantageous from the point of view that
116one does not have to build a second kernel for capturing the dump. But
117at the same time one might want to build a custom dump capture kernel
118suitable to his needs.
119 120Following are the configuration setting required for system and
121dump-capture kernels for enabling kdump support.
122 123System kernel config options
124----------------------------
125 1261) Enable "kexec system call" in "Processor type and features."
127 128 CONFIG_KEXEC=y
129 1302) Enable "sysfs file system support" in "Filesystem" -> "Pseudo
131 filesystems." This is usually enabled by default.
132 133 CONFIG_SYSFS=y
134 135 Note that "sysfs file system support" might not appear in the "Pseudo
136 filesystems" menu if "Configure standard kernel features (for small
137 systems)" is not enabled in "General Setup." In this case, check the
138 .config file itself to ensure that sysfs is turned on, as follows:
139 140 grep 'CONFIG_SYSFS' .config
141 1423) Enable "Compile the kernel with debug info" in "Kernel hacking."
143 144 CONFIG_DEBUG_INFO=Y
145 146 This causes the kernel to be built with debug symbols. The dump
147 analysis tools require a vmlinux with debug symbols in order to read
148 and analyze a dump file.
149 150Dump-capture kernel config options (Arch Independent)
151-----------------------------------------------------
152 1531) Enable "kernel crash dumps" support under "Processor type and
154 features":
155 156 CONFIG_CRASH_DUMP=y
157 1582) Enable "/proc/vmcore support" under "Filesystems" -> "Pseudo filesystems".
159 160 CONFIG_PROC_VMCORE=y
161 (CONFIG_PROC_VMCORE is set by default when CONFIG_CRASH_DUMP is selected.)
162 163Dump-capture kernel config options (Arch Dependent, i386 and x86_64)
164--------------------------------------------------------------------
165 1661) On i386, enable high memory support under "Processor type and
167 features":
168 169 CONFIG_HIGHMEM64G=y
170 or
171 CONFIG_HIGHMEM4G
172 1732) On i386 and x86_64, disable symmetric multi-processing support
174 under "Processor type and features":
175 176 CONFIG_SMP=n
177 178 (If CONFIG_SMP=y, then specify maxcpus=1 on the kernel command line
179 when loading the dump-capture kernel, see section "Load the Dump-capture
180 Kernel".)
181 1823) If one wants to build and use a relocatable kernel,
183 Enable "Build a relocatable kernel" support under "Processor type and
184 features"
185 186 CONFIG_RELOCATABLE=y
187 1884) Use a suitable value for "Physical address where the kernel is
189 loaded" (under "Processor type and features"). This only appears when
190 "kernel crash dumps" is enabled. A suitable value depends upon
191 whether kernel is relocatable or not.
192 193 If you are using a relocatable kernel use CONFIG_PHYSICAL_START=0x100000
194 This will compile the kernel for physical address 1MB, but given the fact
195 kernel is relocatable, it can be run from any physical address hence
196 kexec boot loader will load it in memory region reserved for dump-capture
197 kernel.
198 199 Otherwise it should be the start of memory region reserved for
200 second kernel using boot parameter "crashkernel=Y@X". Here X is
201 start of memory region reserved for dump-capture kernel.
202 Generally X is 16MB (0x1000000). So you can set
203 CONFIG_PHYSICAL_START=0x1000000
204 2055) Make and install the kernel and its modules. DO NOT add this kernel
206 to the boot loader configuration files.
207 208Dump-capture kernel config options (Arch Dependent, ppc64)
209----------------------------------------------------------
210 2111) Enable "Build a kdump crash kernel" support under "Kernel" options:
212 213 CONFIG_CRASH_DUMP=y
214 2152) Enable "Build a relocatable kernel" support
216 217 CONFIG_RELOCATABLE=y
218 219 Make and install the kernel and its modules.
220 221Dump-capture kernel config options (Arch Dependent, ia64)
222----------------------------------------------------------
223 224- No specific options are required to create a dump-capture kernel
225 for ia64, other than those specified in the arch independent section
226 above. This means that it is possible to use the system kernel
227 as a dump-capture kernel if desired.
228 229 The crashkernel region can be automatically placed by the system
230 kernel at run time. This is done by specifying the base address as 0,
231 or omitting it all together.
232 233 crashkernel=256M@0
234 or
235 crashkernel=256M
236 237 If the start address is specified, note that the start address of the
238 kernel will be aligned to 64Mb, so if the start address is not then
239 any space below the alignment point will be wasted.
240 241 242Extended crashkernel syntax
243===========================
244 245While the "crashkernel=size[@offset]" syntax is sufficient for most
246configurations, sometimes it's handy to have the reserved memory dependent
247on the value of System RAM -- that's mostly for distributors that pre-setup
248the kernel command line to avoid a unbootable system after some memory has
249been removed from the machine.
250 251The syntax is:
252 253 crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset]
254 range=start-[end]
255 256 'start' is inclusive and 'end' is exclusive.
257 258For example:
259 260 crashkernel=512M-2G:64M,2G-:128M
261 262This would mean:
263 264 1) if the RAM is smaller than 512M, then don't reserve anything
265 (this is the "rescue" case)
266 2) if the RAM size is between 512M and 2G (exclusive), then reserve 64M
267 3) if the RAM size is larger than 2G, then reserve 128M
268 269 270 271Boot into System Kernel
272=======================
273 2741) Update the boot loader (such as grub, yaboot, or lilo) configuration
275 files as necessary.
276 2772) Boot the system kernel with the boot parameter "crashkernel=Y@X",
278 where Y specifies how much memory to reserve for the dump-capture kernel
279 and X specifies the beginning of this reserved memory. For example,
280 "crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory
281 starting at physical address 0x01000000 (16MB) for the dump-capture kernel.
282 283 On x86 and x86_64, use "crashkernel=64M@16M".
284 285 On ppc64, use "crashkernel=128M@32M".
286 287 On ia64, 256M@256M is a generous value that typically works.
288 The region may be automatically placed on ia64, see the
289 dump-capture kernel config option notes above.
290 291Load the Dump-capture Kernel
292============================
293 294After booting to the system kernel, dump-capture kernel needs to be
295loaded.
296 297Based on the architecture and type of image (relocatable or not), one
298can choose to load the uncompressed vmlinux or compressed bzImage/vmlinuz
299of dump-capture kernel. Following is the summary.
300 301For i386 and x86_64:
302 - Use vmlinux if kernel is not relocatable.
303 - Use bzImage/vmlinuz if kernel is relocatable.
304For ppc64:
305 - Use vmlinux
306For ia64:
307 - Use vmlinux or vmlinuz.gz
308 309 310If you are using a uncompressed vmlinux image then use following command
311to load dump-capture kernel.
312 313 kexec -p <dump-capture-kernel-vmlinux-image> \
314 --initrd=<initrd-for-dump-capture-kernel> --args-linux \
315 --append="root=<root-dev> <arch-specific-options>"
316 317If you are using a compressed bzImage/vmlinuz, then use following command
318to load dump-capture kernel.
319 320 kexec -p <dump-capture-kernel-bzImage> \
321 --initrd=<initrd-for-dump-capture-kernel> \
322 --append="root=<root-dev> <arch-specific-options>"
323 324Please note, that --args-linux does not need to be specified for ia64.
325It is planned to make this a no-op on that architecture, but for now
326it should be omitted
327 328Following are the arch specific command line options to be used while
329loading dump-capture kernel.
330 331For i386, x86_64 and ia64:
332 "1 irqpoll maxcpus=1 reset_devices"
333 334For ppc64:
335 "1 maxcpus=1 noirqdistrib reset_devices"
336 337 338Notes on loading the dump-capture kernel:
339 340* By default, the ELF headers are stored in ELF64 format to support
341 systems with more than 4GB memory. On i386, kexec automatically checks if
342 the physical RAM size exceeds the 4 GB limit and if not, uses ELF32.
343 So, on non-PAE systems, ELF32 is always used.
344 345 The --elf32-core-headers option can be used to force the generation of ELF32
346 headers. This is necessary because GDB currently cannot open vmcore files
347 with ELF64 headers on 32-bit systems.
348 349* The "irqpoll" boot parameter reduces driver initialization failures
350 due to shared interrupts in the dump-capture kernel.
351 352* You must specify <root-dev> in the format corresponding to the root
353 device name in the output of mount command.
354 355* Boot parameter "1" boots the dump-capture kernel into single-user
356 mode without networking. If you want networking, use "3".
357 358* We generally don' have to bring up a SMP kernel just to capture the
359 dump. Hence generally it is useful either to build a UP dump-capture
360 kernel or specify maxcpus=1 option while loading dump-capture kernel.
361 362Kernel Panic
363============
364 365After successfully loading the dump-capture kernel as previously
366described, the system will reboot into the dump-capture kernel if a
367system crash is triggered. Trigger points are located in panic(),
368die(), die_nmi() and in the sysrq handler (ALT-SysRq-c).
369 370The following conditions will execute a crash trigger point:
371 372If a hard lockup is detected and "NMI watchdog" is configured, the system
373will boot into the dump-capture kernel ( die_nmi() ).
374 375If die() is called, and it happens to be a thread with pid 0 or 1, or die()
376is called inside interrupt context or die() is called and panic_on_oops is set,
377the system will boot into the dump-capture kernel.
378 379On powerpc systems when a soft-reset is generated, die() is called by all cpus
380and the system will boot into the dump-capture kernel.
381 382For testing purposes, you can trigger a crash by using "ALT-SysRq-c",
383"echo c > /proc/sysrq-trigger" or write a module to force the panic.
384 385Write Out the Dump File
386=======================
387 388After the dump-capture kernel is booted, write out the dump file with
389the following command:
390 391 cp /proc/vmcore <dump-file>
392 393You can also access dumped memory as a /dev/oldmem device for a linear
394and raw view. To create the device, use the following command:
395 396 mknod /dev/oldmem c 1 12
397 398Use the dd command with suitable options for count, bs, and skip to
399access specific portions of the dump.
400 401To see the entire memory, use the following command:
402 403 dd if=/dev/oldmem of=oldmem.001
404 405 406Analysis
407========
408 409Before analyzing the dump image, you should reboot into a stable kernel.
410 411You can do limited analysis using GDB on the dump file copied out of
412/proc/vmcore. Use the debug vmlinux built with -g and run the following
413command:
414 415 gdb vmlinux <dump-file>
416 417Stack trace for the task on processor 0, register display, and memory
418display work fine.
419 420Note: GDB cannot analyze core files generated in ELF64 format for x86.
421On systems with a maximum of 4GB of memory, you can generate
422ELF32-format headers using the --elf32-core-headers kernel option on the
423dump kernel.
424 425You can also use the Crash utility to analyze dump files in Kdump
426format. Crash is available on Dave Anderson's site at the following URL:
427 428http://people.redhat.com/~anderson/ 429 430 431To Do
432=====
433 4341) Provide relocatable kernels for all architectures to help in maintaining
435 multiple kernels for crash_dump, and the same kernel as the system kernel
436 can be used to capture the dump.
437 438 439Contact
440=======
441 442Vivek Goyal (vgoyal@in.ibm.com)
443Maneesh Soni (maneesh@in.ibm.com)
444 445